1. Field of the Invention
The present invention generally concerns improvements to voltage regulators, specifically including voltage regulators that are used to regulate alternators on vehicles.
The present invention particularly concerns improvements to vehicular electronic voltage regulators used in heavy duty, high current, high performance applications.
Because the main design intent of the present invention is concerned with the high-reliability voltage regulation of vehicular alternators deployed in heavy duty applications, the improvements of the present invention will be seen to be particularly useful for (i) revenue-generating vehicles such as trucks and buses, and (ii) emergency vehicles such as ambulances and fire trucks, supporting large electrical loads.
2. Description of Prior Art
Two basic design technologies have dominated the electronic voltage regulator art: (i) Frequency-On-Demand and (ii) Pulse-Width-Modulation.
2.1 Frequency-On-Demand
Frequency-On-Demand regulation of alternator voltage is by far the oldest, simplest, cheapest and most widely used design. Frequency-On-Demand voltage regulation provides adequate performance in most automotive and light duty applications.
Frequency-On-Demand voltage regulation works on the principle that an error signal is generated by comparing the vehicle""s electrical system voltage to an internal reference in an error amplifier stage to produce an error signal. This error signal is then further amplified and used to drive an output stage of the voltage regulator. The regulator""s output stage in turn controls the current through the alternator""s field winding. An interaction between the output stage and the error amplifier stage (normally through an RC network), produces a sharply defined rectangular pulse-train whose duty cycle determines the excitation level in the alternator""s field winding and hence, the current generating capacity of the alternator.
Due to increasing use of electrical power in vehicles, alternators have become larger, resulting in increased response time due to the very large field inductances and currents involved. When the response time of a large alternator is added to the response time of a commensurately robust voltage regulator itself, then a noticeable and objectionable voltage/current low-frequency fluctuation is observed to occur at the alternator""s output. This phenomena is most noticeable both at no-load and near full-load conditions of the alternator.
In an attempt to deal with this problem, the modern Frequency-on-Demand voltage regulator is designed with a fast response time. The problem with this avenue of improvement has been that when the ON-OFF-ON transitions of the control pulse-train of the alternator are made to occur faster then the level of electrical noise introduced into the vehicle""s electrical system is increased.
With the present (circa 2002) massive use of computers in vehicles, the demand for electrically clean, stable voltage in the vehicle""s electrical system has become a driving factor forcing improvements over the traditional Frequency-on-Demand approach to voltage regulation.
2.2 Pulse-Width-Modulation
In Pulse-Width-Modulation regulation of alternator voltage, a constant frequency pulse-train is internally generated by the voltage regulator and the duty cycle (or pulse width) is then modulated to vary the excitation level in the alternator""s field winding. Since the field-driving pulse-train has a constant frequency that does not depend on external variables such as the field inductance and current flowing into the field, this technology eliminates the low-frequency fluctuations of the Frequency-on-Demand systems. Also, rise and fall times of the ON-OFF-ON transitions can be made sufficiently slow to insert a minimum of electrical noise into the vehicle""s electrical system.
Alas, these improvements result in both higher cost and higher complexity. Thus, although Pulse-Width-Modulation is a flexible and powerful approach to voltage regulation, up until present, circa 2002, cost and complexity issues have kept this technology available only in those applications that have demanded the highest levels of performance.
The present inventionxe2x80x94which has been fully and thoroughly tested by its inventorsxe2x80x94contemplates modifications to proven and highly-reliable Frequency-on-Demand voltage regulators towards the goal of modifying their performance so as to alter their operating mode from Frequency-on-Demand to constant-frequency Pulse-Width-Modulation.
The modifications required to accomplish this objective are as follows:
An xe2x80x9cOUTPUTxe2x80x9d stage of the voltage regulator is reconfigured as a bi-stable multivibrator (also known in electronics engineering as a xe2x80x9cflip-flopxe2x80x9d). This flip-flop circuit also provides an efficient and economical short-circuit protection function for the output power transistor.
An xe2x80x9cOSCILLATORxe2x80x9d, implemented with any of a number of technologies both discrete and integrated, provides a xe2x80x9cRESETxe2x80x9d pulse to the flip-flop with a pre-defined pulse width and pulse period. This xe2x80x9cRESETxe2x80x9d pulse initiates the xe2x80x9cONxe2x80x9d period of the xe2x80x9cOUTPUTxe2x80x9d stage.
An xe2x80x9cERROR AMPLIFIERxe2x80x9d stage of conventional construction rapidly switches the xe2x80x9cOUTPUTxe2x80x9d stage xe2x80x9cOFFxe2x80x9d when system voltage synchronously reaches a preset value, thus terminating the xe2x80x9cONxe2x80x9d period.
Incidental to this construction, the total capacitance across the gate-source terminals of the output stage transistorxe2x80x94normally an MOS power transistorxe2x80x94is selected and is intentionally used to slow down the negative- and positive-going transitions of the transistor to introduce a minimum of electrical noise into the vehicle""s electrical system.
The collective modifications to a Frequency-on-Demand voltage regulator required to so improve its performance by making it into a Pulse-Width-Modulation voltage regulator do not significantly affect the existing (i) reliability or (ii) complexity of this voltage regulator. Indeed, Pulse-Width-Modulation voltage regulators in accordance with the present invention have a substantially similar (i) parts count, and (ii) reliability, to those current (circa 2002) state-of-the-art Frequency-on-Demand regulators which they serve to replace.
Furthermore, the cost of a Pulse-Width-Modulation voltage regulator in accordance with the present invention is only but slightly altered from the Frequency-on-Demand type that it supplants. This is the case even though the performance of a Pulse-Width-Modulation voltage regulator in accordance with the present invention realizes all the valuable features of a true Pulse-Width-Modulation electronic voltage regulator of considerably more complex design.
1. An Electronic Voltage Regulator
Accordingly, in one of its aspects the present invention can be considered to be embodied in an electronic voltage regulator for regulating an alternator to maintain constant a system voltage where a flip-flop, sensitive to a xe2x80x9cRESETxe2x80x9d pulse to initiate an xe2x80x9cONxe2x80x9d condition, serves as an xe2x80x9cOUTPUTxe2x80x9d stage of the voltage regulator.
An oscillator produces a xe2x80x9cRESETxe2x80x9d pulse having a pre-defined pulse width and pulse period, and provides this xe2x80x9cRESETxe2x80x9d pulse to the xe2x80x9cOUTPUTxe2x80x9d stage flip-flop.
Meanwhile, an xe2x80x9cERROR AMPLIFIERxe2x80x9d stage synchronously switches the xe2x80x9cOUTPUTxe2x80x9d stage flip-flop xe2x80x9cOFFxe2x80x9d when the system voltage reaches a preset value, thus terminating the xe2x80x9cONxe2x80x9d period of the xe2x80x9cOUTPUTxe2x80x9d stage flip-flop.
By these components, and these connections, the oscillator, the error amplifier and the output stage flip-flop produce in combination a pulse-width-modulated signal.
The OUTPUT stage flip-flop may in particular include a Field Effect Transistor (FET) having gate and source terminals. The parasitic capacitance of the gate-source terminals of this FETxe2x80x94either acting alone or in combination with an external discrete capacitorxe2x80x94serves to slow both negative- and positive-going transitions of the OUTPUT stage flip-flop, therein introducing a minimum of electrical noise into an electrical system in which the electronic voltage regulator is used.
The oscillator may in particular be realized as a two-transistor equivalent to a Programmable Unijunction Transistor (PUT), being two transistors with one transistor connected backwards to the other with its emitter and its collector interchanged so as to lower gain and avoid PUT latching. This unusual construction conserves components, and is thus economical.
In yet another unusual use of components, two capacitors are used as a voltage divider in the error amplifier stage, with a signal from one capacitor being fed forward to the output amplifier. This signal accelerates the xe2x80x9cONxe2x80x9d and xe2x80x9cOFFxe2x80x9d transitions in the OUTPUT stage. Furthermore, the capacitor providing this signal also implements a xe2x80x9cvoltage bleederxe2x80x9d function on an integrating capacitor of the error amplifier stage, assuring that a RESET pulse xe2x80x9cbleeds awayxe2x80x9d a small fraction of the voltage stored on this integrating capacitor (by virtue of capacitive divider effect) and assuring proper synchronization between the xe2x80x9cERROR AMPLIFIERxe2x80x9d stage and the xe2x80x9cOUTPUTxe2x80x9d stage, permitting the integrating capacitor to be reset at the start of each xe2x80x9cONxe2x80x9d cycle to a smaller voltage by virtue of the RESET pulse. Noise immunity, as well as response time, is thus improved.
2. An Improvement to an Electronic Voltage Regulator So as to Cause It to Produce a Pulse-width-modulated Signal
In another of its aspects the present invention can be considered to be embodied in an improvement to an electronic voltage regulator so as to cause the voltage regulator to produce a pulse-width-modulated signal.
The improvement is to an electronic voltage regulator producing a frequency-on-demand signal across the field winding of an alternator, the voltage regulator being connected across a battery that develops a voltage between a positive terminal and a negative terminal. This conventional voltage regulator has, among other parts, (1) an error amplifier, having a signal input port and a signal output port, selectively conducting respective to variations in the voltage of the battery so as to produce a control signal, which error amplifier is connected at its signal output port to (2) an output transistor responsive to the control signal for selectively conducting so as to produce at its output port the frequency-on-demand signal.
To this electronic voltage regulator the improvement of the present invention adds an oscillator producing and providing a xe2x80x9cRESETxe2x80x9d pulse having a pre-defined pulse width and pulse period. Meanwhile, the output transistor is used as one leg of a flip-flop. This flip-flop is sensitive to (i) the xe2x80x9cRESETxe2x80x9d pulse from the oscillator to initiate an xe2x80x9cONxe2x80x9d condition of the flip-flop (where its output transistor conducts), and to (ii) the error amplifier stage forxe2x80x94when the system voltage synchronously reaches a preset valuexe2x80x94switching the flip-flop to an xe2x80x9cOFFxe2x80x9d condition (where its output transistor does not conduct). The xe2x80x9cONxe2x80x9d and the xe2x80x9cOFFxe2x80x9d periods of the flip-flop accordingly alternate.
By this construction, this connection, and this coaction (i) the oscillator, (ii) the error amplifier and (iii) the flip flop of which a one leg is the output transistor, produce in combination the pulse-width-modulated signal.
The oscillator is preferably a two-transistor equivalent to a Programmable Unijunction Transistor (PUT). Namely, it is two transistors with one transistor connected backwards to the other with its emitter and its collector interchanged so as to lower gain and avoid PUT latching.
3. A Method of Regulation of a System Voltage Produced by an Alternator
In yet another of its aspects the present invention can be considered to be embodied in a method of the regulation of a system voltage produced by an alternator having a field coil.
The method consists of controlling field coil current to the alternator through a bi-stable multivibrator, also called a flip-flop.
Resetting the flip flop to provide the field coil current is realized by and with a reset pulse of pre-defined pulse width and pulse period. This reset pulse is produced by an oscillator.
Setting the flip flop to turn off field coil current to the alternator is realized with and by a signal synchronously produced in an error amplifier when voltage produced by the alternator reaches a preset value.
By this method a constant-frequency, pulse-width-modulated, output signal is realized.
These and other aspects and attributes of the present invention will become increasingly clear upon reference to the following drawings and accompanying specification.